Coniferous woods, especially pine, contain, in addition to cellulose and hemicellulose, such components as lignin, resin acids and long-chain fatty acids. Crude tall oil, a mixture of the resin and fatty acids, is produced as a chemical by-product of the Kraft pulping process.
In the Kraft pulping process, wood chips are fed into a digester where a "white liquor" containing sodium hydroxide (NaOH) and sodium sulfide (Na.sub.2 S) is added. The contents are then heated according to a predetermined schedule to complete the cooking reactions, wherein the resin acids and fatty acids are released from the wood chips and saponified. The resulting cooked wood pulp is separated from the residual cooking liquor, known as "black liquor", by filtration to yield wood pulp referred to as "brown stock". The brown stock wood pulp is treated further in the papermaking process to produce papers of various grades. The sodium salts of the resin acids and fatty acids, commonly referred to as tall oil soap or "black liquor soap", are suspended in the remaining black liquor.
The recovery of various chemicals from the black liquor, the reconstitution of those chemicals to form fresh cooking liquors, the realization of energy from incineration of organic residuals, and minimization of air and water pollution, are all vital parts of the Kraft process. Among those chemicals which are recovered from the black liquor is tall oil soap. The soap is contained in the "weak" (i.e., unconcentrated) black liquor which is recovered in the pulping process as a filtrate from the separation of the pulp (brown stock).
The initial weak black liquor (at about 15% solids content) is filtered to remove fiber (brown stock) and is passed into a weak liquor skimmer. About 30% to 70% of the available black liquor soap is skimmed off. The black liquor then passes through several stages of evaporative concentration to raise the solids content to 25-30%. The black liquor then passes through a second skimmer from which the remaining 30-70% of soap is skimmed off. The black liquor finally passes through more stages of evaporative concentration to raise the solids to 65-75%. It is then fed to a recovery furnace to be burned. Burning the organic content of the black liquor provides energy for the paper making process and reconstitutes the inorganic chemicals for reuse in fresh pulping liquors. See, e.g., Foran, C. D., "Black Liquor Soap Recovery Methods Employed by Union Camp," Naval Stores Review, 94 (3), 14-18 (1984), the disclosures of which are incorporated by reference herein in their entirety. One skilled in the art o making Kraft paper will recognize that there are various techniques for recovering tall oil soap from the Kraft black liquor.
The separated tall oil soap is then normally acidulated with sulfuric acid (H.sub.2 SO.sub.4) to form crude tall oil, which can be refined by vacuum fractionation to separate tall oil fatty acids and tall oil rosin. The fatty acids, resin acids, and their derivatives are used in numerous industrial applications, including soaps, lubricants, inks, adhesives, and coatings. The acidulation process generally comprises the addition of H.sub.2 SO.sub.4 to the tall oil soap to produce crude tall oil and sodium sulfate (Na.sub.2 SO.sub.4) salt cake solution. One skilled in the art will recognize that there are various acidulation processes utilizing H.sub.2 SO.sub.4 for the conversion of tall oil soap to crude tall oil. For a more thorough discussion (with references), see, e.g., McSweeney, E. E., "Sulfate Naval Stores", Naval Stores: Production, Chemistry, Utilization, pp. 158-199, Duane Zinkel and James Russell, eds., Pulp Chemicals Association, (N.Y. 1989), the disclosures of which are incorporated by reference herein in their entirety. The Na.sub.2 SO.sub.4 as described above is then recycled to the Kraft pulping process to make up for sodium and sulfur losses.
The economics of the production of crude tall oil from tall oil soap depends to a considerable degree on the ability to utilize of the salt cake from the acidulation process. Heretofore, the producer of crude tall oil would return the by-product Na.sub.2 SO.sub.4 to the paper mill as make-up chemicals in the Kraft pulping process. A typical Kraft mill in the early 1970s was designed for sodium recovery of around 93% and sulfur recovery of only about 61%. See, Twiss, A. H., Naval Stores Review, 94 (2), 14-16 (1984), the disclosures of which are incorporated by reference herein in their entirety. With the introduction of more stringent environmental controls on the emission of air and water pollutants, a new "grass roots" state of the art mill (1984) being designed to meet the more demanding new source performance standards was predicted to achieve over 97% sodium recovery and 91% sulfur recovery. Twiss reports that as of 1984, total sodium loss expressed as equivalent salt cake had been decreased from 135 to 70 pounds/ton of pulp for a mill meeting then existing source standards, down to 53 pounds for a new state of the art (1984) mill. Total sulfur losses were decreased from 175, to 75, to 40 pounds of equivalent salt cake per ton of pulp, due mainly to reduced sulfur emissions from the recovery boiler and lime kiln. These reduced losses have had a drastic impact on the make-up sodium and sulfur requirements in the Kraft pulping process, which balance these losses. The ratio of sulfur to sodium make-up, both expressed as salt cake, declined from about 1.30 in 1970, to about 1.07 in 1984, to about 0.75 currently. This ratio is expected to decline further in the near future. The result is that paper mills no longer desire, or require, the previous levels of salt cake produced in the sulfuric acid acidulation of tall oil soap to make up for losses of sodium and sulfur. In fact, a large amount of the salt cake make-up has been replaced with more costly caustic soda to lower the sulfur make-up, and this amount is increasing. Without the paper mills to use all of the salt cake produced by the acidulation of tall oil soap, the remainder has to be sewered. However, the organic contaminants in a sewer discharge containing salt cake serves to increase the biological oxygen demand (BOD) levels in the water treatment system receiving the discharge. There are, therefore, practical limitations on the discharge of salt cake to the sewer. These limitations are bound to increase due to increasingly stringent environmental restrictions.
A number of other authors have also discussed tall oil soap acidulation and sulfur balance problems in Kraft mills. See, e.g., Wong, A., Naval Stores Review, 94 (3), 8-10 (1984), the disclosures of which are incorporated by reference herein in their entirety. Wong reported the same problems associated with the salt cake from the acidulation of the tall oil soap as did Twiss. Wong proposed the replacement of H.sub.2 SO.sub.4 used in soap acidulation with chlorine dioxide generator effluent to lower the sulfur input into the mill liquor system. However, the reduced sulfur input would still be higher than the losses in the mill by about 10%.
It has been suggested that the tall oil acidulation process can be modified to reduce or eliminate by-product sulfur by the use of other common acids, such as hydrochloric. The use of such acids, however, would prohibit recovery of the spent acid due to build-up of extraneous anions and problems in tall oil purification.
Electrolysis of spent acid to NaOH and a mixture of H.sub.2 SO.sub.4 and Na.sub.2 SO.sub.4 has also been investigated.
As also reported by Twiss, at p. 16, replacement of about one-half of the H.sub.2 SO.sub.4 with carbon dioxide has been described by Bills, U. S. Pat. No. 3,901,869 (1975), the disclosures of which are incorporated by reference herein in their entirety. Specifically, Bills describes the acidification of tall oil soap using a water to soap weight ratio between 0.75 and 2.00, a temperature between ambient and 120.degree. F. (49.degree. C.), and sufficient carbon dioxide to lower the pH to between 7-8. In accordance with Bills, the product separated into an upper layer containing tall oil acids and unreacted soaps, and a lower layer containing aqueous sodium bicarbonate. To complete the recovery of tall oil acids, the upper layer required further reaction with 0.0974 to 0.150 pounds of H.sub.2 SO.sub.4 per pound of crude tall oil. The Bills process resulted in only a 40-53% reduction in the amount of H.sub.2 SO.sub.4 needed to complete the conversion to tall oil soap using a single stage CO.sub.2 process. Thus, Bills also proposed a two-stage process, in which the previously carbonated soap-acid layer is then mixed with an equivalent amount of fresh water and acidified again with CO.sub.2. The two-step process described by Bills, however, has the disadvantage of necessitating, as Bills pointed out, "an increase in the use of water which would add greatly to the evaporation load".
Vardell, Jr., U.S. Pat. No. 4,075,188 (1978) describes a somewhat improved carbon dioxide acidification reaction using a water-immiscible solvent. The best example showed that only 60% of the tall oil salts were converted to free acids in a single stage reaction using carbon dioxide at 500 psig, 150.degree. C., and a 1.0 to 0.33 to 0.67 weight ratio of soap to water to solvent. It should also be noted that the use of a solvent, as taught by Vardell, Jr., itself poses additional environmental concerns.
These proposed solutions only partially address the problem. Very simply, the most effective and efficient processes for acidulation of tall oil soap to crude tall oil disclosed to date involve the use of large amounts of H.sub.2 SO.sub.4 as the sole acidulation agent. This, however, results in far too much Na.sub.2 SO.sub.4 being produced in soap acidulation plants for convenient recycling in the paper mill liquor system, or for environmentally safe and economical disposal.
There is a long felt need in the papermaking industry for a process to acidulate tall oil soap to form crude tall oil which can significantly reduce the formation of Na.sub.2 SO.sub.4 salt cake. This need has not been satisfied to date, even in the face of a compelling urgency caused by more stringent environmental emission controls and reduced requirements for salt cake in the paper mills.
The present invention relates to a process for the acidulation of tall oil soap to form crude tall oil which reduces the use of H.sub.2 SO.sub.4 in the acidulation process. These and other objects of the present invention will become readily apparent from the subject specification.